This invention relates to a control system for hydraulic pumps of a hydraulic type civil machine.
In a conventional hydraulic type civil machine, for example in a conventional hydraulic power shovel, it is so constructed that a small number of hydraulic pumps drive such equipment as a boom cylinder 2, an arm cylinder 3, a bucket cylinder 4, a slewing motor 5 and a travelling motor 6 as shown in FIG. 1, whereby controlling working tools such as a boom 7, an arm 8 and a bucket 9 as well as controlling slewing and travelling of these working tools, and for the hydraulic circuit therein parallel circuits have been usually employed. As a result, hydraulic power loss is a considerable amount while an operating lever is set to the neutral position. In another conventional hydraulic power shovel, in order to meet a recent trend in which machines become large in size and in order to output hydraulic power equal to load being charged, a variable pump 11 is employed as shown in FIG. 2. In such a machine, an engine 10 drives a variable pump 11 and a control pump 12, and the control pump 12, in turn, actuates a mechanical cylinder 13 to control an inclination angle of a swash plate of the variable pump 11, thereby controlling flow rate of hydraulic operating oil to be fed into a manually operated directional control valve 14. The manually operated directional control valve 14 controls a working tool cylinder 16. Maximum pressure P.sub.2 and maximum flow rate Q.sub.2 (FIG. 3) are restricted by a relief valve 15 and the variable pump 11, respectively.
However, in such conventional control systems as described above, since they are of manual operation type, relief loss of oil pressure is very large when a hydraulic cylinder lies in the stroke end or when overload is charged during excavation. In this connection, referring to FIG. 3 and 4 which show relationship between pressure P and flow rate Q of hydraulic power, assign reference alphanumerals PS.sub.N and PS.sub.R to hydraulic power loss at the neutral condition and at an overload condition, respectively, then these values PS.sub.N and PS.sub.R are expressed by the following equations. ##EQU1##
Large relief loss of oil pressure causes temperature rise in hydraulic operational oil, which results in deterioration of hydraulic oil used as well as a higher rate of fuel consumption.
As described above, in a conventional hydraulic type civil machine, for example in a conventional hydraulic power shovel, a single engine drives a plurality of hydraulic pumps, which in turn drive a plurality of hydraulic motors and cylinders, thereby performing travelling and slewing of the machine as well as various kinds of excavation work.
Such a conventional hydraulic type civil machine is likely to suffer engine failure when raising the delivery pressure of the engine beyond the rated output capacity of the engine or using a plurality of hydraulic pumps simultaneously with their delivery pressure raised.
In order to prevent engine failure in such conventional power shovel, an operator must foresee it by always paying attention to troubled signs such as abnormal engine noise and engine speed reduction. Upon recognizing any troubled sign, the operator must return the operating lever to the neutral position so as to reduce the work load.
However, such system in which an operator foresees engine failure as described above is disadvantageous in that it depends upon operator's senses and therefore frequency of the engine failure depends upon operator's skill and work efficiency is lowered as well as operators are exhausted.